A capacitive coupled (parallel plate) plasma processing apparatus, an inductively coupled plasma processing apparatus, a microwave plasma processing apparatus or the like has been put to practical use in an apparatus for performing a micro process such as an etching process or a film forming process on a target substrate to be processed by using a plasma. Especially, in the parallel plate plasma processing apparatus, a radio frequency (RF) power is supplied to at least one of an upper electrode and a lower electrode facing to each other, and a gas is excited by an energy of an electric field of the RF power to thereby generate a plasma. Then, the generated discharge plasma is used for performing such micro process on the target substrate.
In response to recent demands for the device scaling-down, it has been necessary to supply a power having a relatively high frequency and generate a high-density plasma. As shown in FIG. 9, if the frequency of a power supplied from a RF power supply 915 becomes increased, a current of the RF power is propagated through the surface of a lower electrode 910 and from an end portion of an upper surface of the lower electrode 910 toward a central portion thereof.
Accordingly, the intensity of the electric field gets higher at the central portion of the lower electrode 910 than that at the end portion thereof and, thus, the ionization and dissociation of the gas is more accelerated at the central portion of the lower electrode 910 than that at the end portion thereof. As a result, the electron density of the plasma gets higher at the central portion of the lower electrode 910 than that at the end portion thereof. Since the resistivity of the plasma is lowered at the central portion of the lower electrode 910 where the electron density of the plasma is higher than the end portion, the RF current is also concentrated at a central portion of an upper electrode 905 that faces the lower electrode 910, thereby making the plasma density further non-uniform.
For that reason, to improve the uniformity of the plasma, there has been suggested a method of installing a flat dielectric body 920a at the lower center of the upper electrode 905 as shown in FIG. 2C (see, e.g., Japanese Patent Application Publication No. 2004-363552). With such method, the electric field intensity distribution E/Emax under the dielectric body 920a may be lowered by the action of the dielectric body 920a. 
Further, to improve the uniformity of the plasma further, there has been suggested a method of forming the dielectric body 920 in a taper shape as shown in FIG. 2B. With such method, since a capacitance component becomes larger at an end portion of the dielectric body 920 than that at a central portion thereof, the electric field intensity distribution E/Emax may not be excessively lowered at the end portion of the dielectric body 920 as compared with the case of the flat dielectric body 920a shown in FIG. 2C. Resultantly, it is possible to improve the uniformity of the electric field intensity distribution E/Emax more efficiently.
In the meantime, in the case of installing the taper-shaped dielectric body 920 in a base member (of the upper electrode 905), the dielectric body 920 is adhered to the base member by using an adhesive or screws. At this time, a linear thermal expansion difference may develop between the base member formed of, e.g., a metal such as aluminum or the like and the dielectric body 920 formed of, e.g., a ceramic or the like. Accordingly, it is necessary to provide an adequate gap therebetween.
However, in the case of the dielectric body 920 of the taper shape, the dimensional precision at the tapered portion may be deteriorated due to the machining work and, resultantly, the stress concentration occurs due to such thermal expansion difference. In addition, the stress concentration also occurs due to a thermal conductive difference caused by a deviation of dimensional tolerance of the adhesive interface and/or a difference of thickness of the dielectric body 920. Such stress concentration causes the adhesive on the adhesive interface to be peeled off and the peeled-off adhesive comes out of the gap, thereby polluting the inside of the chamber.